Conventional screen-film radiography does not allow high contrast visualisation of both the lung and mediastinal regions of the chest due predominantly to the large radiographic attenuation differences between lung and mediastinal regions of the chest and to the significant scatter component that exits from the mediastinum. The extent of radiographic attenuation differences between the lung and mediastinum has been measured and studies of the scatter component of the exit beam from the chest have been recorded. The scatter-to-primary ratio exiting from the mediastinum is almost ten times greater than that which has been measured exiting from the lungs. Large inter-patient variations for both mediastinum and lung regions have also been noted.
Various attempts have been made to resolve this problem, including the use of beam filtration devices, beam scanning devices, improved imaging receptors and digital imaging techniques.
Also, systems have been proposed and developed that involve the use of more than one film to capture an image from a single exposure in a number of cases, including in tomography, dentistry, mammography, chest and intensive care situations. A tomographic "book cassette" employs multiple screen-film combinations but suffers from reduced image quality due to increased scatter on the lower films in the cassette and from difficulty in equalising the radiographic density on all films.
Double-loading of two films in a single screen pair has been employed for producing two identical films. Furthermore, two films have been used in dental radiography as a method of halving the patient dose, and a thin tin foil has been located between two unscreened film packs to permit viewing of the skin line in mammography. Also, a dual screen-film cassette system has been used to expand the dynamic range in chest radiography, but without any filter being used.
However, none of the above approaches has provided for high contrast visualisation of regions, such as the lung and mediastinal regions of the chest, that exhibit different attentuating characteristics.